Long-term reactions of visual pigments to light, darkness and hormones. Biochemical and photobiochemical relationships between the pigment epithelium and retinal photoreceptors. Particular regard is given to the interchange and storage of retinols ("vitamins A1 and A2") their esters and their isomers between these structures during light and darkness, and the role of the pigment epithelium in pigment regeneration and synthesis. Study of the evolution of visual systems, with particular emphasis on developments resulting from isolations of fish species during and after the last glacial epoch. This study could lead to a better understanding of the structure of visual pigment proteins (opsins) and the enzymes associated with the synthesis of visual pigment prosthetic groups (retinal and 3-dehydroretinal).